Geldanamycin is a biologically active ansa ring compound derived from Streptomyces hygroscopicus, var. Geldanus, var. nova, [DeBoer, et al., J.Antibiot., 23, 442 (1970) and see, generally, Rinehart, Accounts Chem. Res., 5, 57 (1972)], having a structure as set out below. ##STR2##